I85PREP
⌘ K
Florida Biology EOC study materials
Florida Biology EOC study resources

Florida Biology EOC practice questions

One question at a time. Build your Florida Biology EOC understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Molecular and Cell Biology

A student views a wet-mount slide of onion epidermis and cannot see the cell walls clearly. Which adjustment most directly improves the contrast of these unstained, transparent cells?

  1. Rotate the coarse adjustment knob while watching the eyepiece
  2. Switch from the high-power objective to the scanning objective
  3. Move the slide toward the right edge of the stage clips
  4. Close the iris diaphragm to reduce the light passing through the specimen Correct answer.

Explanation

Unstained cells are nearly transparent, so their refractive differences appear only when light is limited. Closing the iris diaphragm increases contrast, which makes thin walls and nuclei visible. Changing magnification or moving the slide does not address the lack of contrast.

Question 2Molecular and Cell Biology

Which statement best describes the role of the cell membrane in maintaining homeostasis for a single-celled organism?

  1. It converts light energy into chemical bond energy
  2. It regulates which substances enter and leave the cell Correct answer.
  3. It synthesizes proteins that repair the cell wall
  4. It stores genetic instructions used during cell division

Explanation

The plasma membrane is selectively permeable, so it controls the movement of ions, water, and nutrients while keeping wastes balanced. This selective control is a key part of maintaining a stable internal environment. Protein synthesis, genetic storage, and photosynthesis occur at other structures.

Question 3Molecular and Cell Biology

A plant cell is placed in a hypotonic solution. Which observation is expected after several minutes?

  1. The central vacuole takes in water and turgor pressure rises Correct answer.
  2. The chloroplasts dissolve as the cell loses structural support
  3. The cell membrane pulls away from the cell wall as water exits
  4. The cell shrinks because solutes move out of the cytoplasm

Explanation

In a hypotonic solution the outside solute concentration is lower than inside the cell, so water enters by osmosis. In plant cells the incoming water fills the central vacuole and presses the membrane against the rigid wall, producing turgor pressure. Loss of water and shrinking would occur in a hypertonic solution.

Question 4Molecular and Cell Biology

Which molecule serves as the primary energy currency that cells use to power endergonic reactions?

  1. DNA
  2. Glucose
  3. Cholesterol
  4. ATP Correct answer.

Explanation

ATP releases usable energy when its terminal phosphate bond is hydrolyzed, and that energy drives reactions that would not occur on their own. Glucose stores energy but must first be broken down through cellular respiration to regenerate ATP. DNA and cholesterol serve informational and structural roles rather than energy transfer.

Question 5Molecular and Cell Biology

Which organelle is the primary site of ATP production through aerobic cellular respiration in a eukaryotic cell?

  1. Lysosome
  2. Mitochondrion Correct answer.
  3. Ribosome
  4. Golgi apparatus

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which generate most of the cell's ATP during aerobic respiration. Ribosomes build proteins, the Golgi modifies and packages them, and lysosomes digest materials. These other organelles do not carry out oxidative ATP synthesis.

Question 6Molecular and Cell Biology

Which property of the phospholipid bilayer allows small nonpolar molecules such as oxygen to cross a membrane readily?

  1. A charged phosphate region facing the membrane interior
  2. A continuous sheet of transport proteins covering the surface
  3. A rigid layer of cellulose fibers embedded in the membrane
  4. A hydrophobic interior formed by fatty acid tails Correct answer.

Explanation

Phospholipids arrange with their polar heads facing water and their nonpolar tails pointing inward, creating a hydrophobic core. Nonpolar gases such as oxygen dissolve in and diffuse through this core easily. Proteins are scattered rather than continuous, and phosphate heads face outward toward water.

Question 7Molecular and Cell Biology

An enzyme catalyzes a reaction in the human stomach at about pH 2. If the same enzyme is placed in a buffer at pH 7, the reaction rate drops sharply. Which explanation is most consistent with these results?

  1. The higher pH increases substrate concentration so collisions become less frequent
  2. The enzyme is permanently consumed by the reaction and cannot be reused
  3. The change in pH alters the enzyme's shape and reduces substrate binding at the active site Correct answer.
  4. The buffer removes the activation energy needed for the reaction to proceed

Explanation

Enzyme activity depends on proper folding and on the ionization of amino acids at the active site. A large pH shift can disrupt ionic and hydrogen bonds, changing the active site's shape so the substrate fits poorly. Enzymes are not consumed, and buffers do not eliminate activation energy.

Question 8Molecular and Cell Biology

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. Which net products are also formed in this pathway?

  1. 4 ATP and 4 NADH
  2. 2 ATP and 2 NADH Correct answer.
  3. 2 ATP and 2 FADH2
  4. 36 ATP and 6 CO2

Explanation

Glycolysis uses two ATP to prime the pathway and produces four ATP, giving a net gain of two ATP. It also reduces two NAD+ to two NADH while splitting glucose into two three-carbon pyruvates. FADH2 and large ATP yields arise later in the mitochondria.

Question 9Molecular and Cell Biology

A cell biologist treats cells with a drug that prevents the mitotic spindle from attaching to chromosomes. Which outcome is most likely?

  1. Chromosomes replicate a second time within the same cell cycle
  2. Sister chromatids separate too quickly and form extra nuclei
  3. Chromosomes fail to align at the metaphase plate and division is halted Correct answer.
  4. The nuclear envelope reforms prematurely around unseparated DNA

Explanation

Spindle microtubules attach to kinetochores and pull chromosomes to the cell equator. Without attachment, chromosomes cannot align properly, and the spindle assembly checkpoint delays anaphase. Premature nuclear envelope reformation and extra DNA replication are not direct consequences of failed attachment.

Question 10Molecular and Cell Biology

Which statement correctly distinguishes mitosis from meiosis in a human cell?

  1. Mitosis occurs only in gametes, while meiosis occurs only in body cells
  2. Mitosis produces four haploid cells, while meiosis produces two diploid cells
  3. Mitosis halves the chromosome number, while meiosis keeps it unchanged
  4. Mitosis produces two genetically identical diploid cells, while meiosis produces four genetically varied haploid cells Correct answer.

Explanation

Mitosis separates identical sister chromatids to yield two diploid cells that support growth and repair. Meiosis includes two divisions and crossing over, producing four haploid gametes with new allele combinations. Confusing the chromosome outcomes or the cell types reverses the two processes.

Question 11Molecular and Cell Biology

In a DNA molecule, adenine pairs with thymine and guanine pairs with cytosine. If a sample of double-stranded DNA contains 30% adenine, what percentage of its bases is guanine?

  1. 20% Correct answer.
  2. 40%
  3. 30%
  4. 70%

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 30%, and A plus T accounts for 60% of the bases. The remaining 40% is split equally between guanine and cytosine, making guanine 20%.

Question 12Molecular and Cell Biology

During transcription, which enzyme builds an mRNA strand using one DNA strand as a template?

  1. RNA polymerase Correct answer.
  2. Peptidyl transferase
  3. Helicase
  4. DNA ligase

Explanation

RNA polymerase binds a promoter, unwinds the DNA, and adds complementary RNA nucleotides in the 5' to 3' direction. Ligase joins DNA fragments, helicase unwinds DNA during replication, and peptidyl transferase forms peptide bonds during translation.

Question 13Molecular and Cell Biology

A mutation changes one DNA base but the encoded protein has the same amino acid sequence. Which term best describes this change?

  1. Silent mutation Correct answer.
  2. Nonsense mutation
  3. Missense mutation
  4. Frameshift mutation

Explanation

The genetic code is redundant, so several codons specify the same amino acid. A substitution that produces a synonymous codon leaves the protein unchanged and is called silent. Frameshift and nonsense mutations alter downstream reading or create stop codons, and missense mutations change an amino acid.

Question 14Molecular and Cell Biology

Which process produces the genetic variation that makes each gamete from one parent genetically unique?

  1. Crossing over and independent assortment during meiosis Correct answer.
  2. Replication of DNA during the S phase of interphase
  3. Translation of mRNA by ribosomes in the cytoplasm
  4. Diffusion of ions across the plasma membrane

Explanation

Crossing over exchanges segments between homologous chromosomes, and independent assortment randomly orients homolog pairs at metaphase I. Together these events shuffle alleles into new combinations in gametes. DNA replication copies information faithfully, and translation and diffusion do not reshuffle alleles.

Question 15Molecular and Cell Biology

A freshwater paramecium lives in a hypotonic environment and constantly gains water. Which structure most directly helps it maintain water balance?

  1. Contractile vacuole Correct answer.
  2. Macronucleus
  3. Cilium
  4. Food vacuole

Explanation

The contractile vacuole collects excess water and expels it, preventing the cell from swelling and bursting. Food vacuoles digest nutrients, cilia provide movement, and the macronucleus controls cell functions.

Question 16Molecular and Cell Biology

Which statement best explains why a reaction catalyzed by an enzyme proceeds faster than the same uncatalyzed reaction at the same temperature?

  1. The enzyme increases the free energy released by the reaction
  2. The enzyme changes the reaction into a different chemical pathway with no products
  3. The enzyme raises the temperature of the surrounding solution
  4. The enzyme lowers the activation energy required for the reaction Correct answer.

Explanation

Enzymes stabilize the transition state, which lowers the activation energy barrier and allows more molecules to react per unit time. They do not change the overall free energy difference between reactants and products. Temperature and product identity are not altered by catalysis.

Question 17Molecular and Cell Biology

A cell is placed in a solution containing a solute that cannot cross the membrane, and the solution has a higher solute concentration than the cytoplasm. What happens to the cell?

  1. Solutes move into the cell until concentrations equalize
  2. Water leaves the cell and the cell shrinks Correct answer.
  3. Water enters the cell and the cell swells
  4. Nothing changes because the membrane is impermeable to water

Explanation

Because the outside solution is hypertonic, water moves out of the cell by osmosis toward the higher solute concentration. The cell loses volume and shrinks, a process called crenation in animal cells. The solute cannot cross, and water permeability ensures net movement occurs.

Question 18Molecular and Cell Biology

Which of these best describes the relationship between the light-dependent reactions and the Calvin cycle in photosynthesis?

  1. The light reactions fix carbon dioxide while the Calvin cycle absorbs photons
  2. The light reactions produce ATP and NADPH that the Calvin cycle uses to fix carbon dioxide Correct answer.
  3. The Calvin cycle produces ATP and NADPH that the light reactions use to split water
  4. The two stages are independent and share no molecules

Explanation

Thylakoid membranes host the light reactions, which capture energy and store it in ATP and NADPH while releasing oxygen from water. The stroma hosts the Calvin cycle, which uses that ATP and NADPH to reduce carbon dioxide into sugar. The two stages are linked by these energy carriers.

Question 19Molecular and Cell Biology

A cell has a mutation that prevents the formation of the nuclear pores. Which process would be most directly impaired?

  1. Synthesis of lipids in the smooth endoplasmic reticulum
  2. Transport of mRNA from the nucleus to the cytoplasm Correct answer.
  3. Breakdown of hydrogen peroxide by peroxisomes
  4. Production of ATP in the mitochondria

Explanation

Nuclear pores are channels through the nuclear envelope that allow mRNA and ribosomal subunits to leave the nucleus and let proteins enter. Without pores, this exchange is blocked. Lipid synthesis, ATP production, and peroxide breakdown occur in other organelles and do not require nuclear pores.

Question 20Molecular and Cell Biology

In a laboratory, a researcher measures the rate of an enzyme reaction as substrate concentration increases. The rate rises and then levels off. Which interpretation is best supported?

  1. The reaction has reached equilibrium and no product forms
  2. All active sites are occupied, so the enzyme is saturated and the maximum rate is reached Correct answer.
  3. The enzyme has been permanently denatured by the substrate
  4. The substrate has become a competitive inhibitor of the enzyme

Explanation

At low substrate concentrations, collisions with active sites limit the rate, so adding substrate speeds the reaction. Once every enzyme molecule is bound, additional substrate cannot increase the rate, producing a plateau called saturation. This plateau does not indicate denaturation, inhibition, or equilibrium.

Question 21Molecular and Cell Biology

A mutation deletes one nucleotide near the beginning of a gene's coding region. Which consequence is most likely for the resulting protein?

  1. The mRNA is not transcribed because the promoter is destroyed
  2. Only the deleted nucleotide is missing, and the rest of the protein is unchanged
  3. The reading frame shifts, so most downstream amino acids change and the protein is usually nonfunctional Correct answer.
  4. A single amino acid is replaced, and the rest of the sequence remains identical

Explanation

Codons are read in non-overlapping triplets, so deleting one base shifts every downstream triplet. This frameshift changes most amino acids and often introduces a premature stop codon, usually destroying function. A single substitution, not a deletion, typically changes only one amino acid.

Question 22Molecular and Cell Biology

A cell is treated with a drug that blocks the Krebs cycle but leaves glycolysis and the electron transport chain functional. Which immediate effect is most likely?

  1. Glycolysis stops immediately because glucose cannot be broken down
  2. Oxygen consumption increases because the cycle is bypassed
  3. NADH and FADH2 production from the cycle falls, reducing ATP yield from oxidative phosphorylation Correct answer.
  4. The electron transport chain produces more ATP because fewer electrons arrive

Explanation

The citric acid cycle generates NADH and FADH2 that donate electrons to the electron transport chain. Blocking the cycle reduces these carriers, so oxidative phosphorylation produces less ATP. Glycolysis can continue independently, and fewer electrons would not increase ATP output.

Question 23Molecular and Cell Biology

Two enzymes act on the same substrate, but enzyme A has a much lower Michaelis constant (Km) than enzyme B. What does this difference indicate about the enzymes?

  1. Enzyme B requires a cofactor that enzyme A does not
  2. Enzyme A has a lower maximum velocity than enzyme B
  3. Enzyme A reaches half-maximal velocity at a lower substrate concentration, indicating a higher substrate affinity Correct answer.
  4. Enzyme B binds substrate more tightly at all concentrations

Explanation

Km is the substrate concentration at which the reaction reaches half of its maximum velocity. A lower Km means the enzyme achieves half-maximal rate with less substrate, reflecting tighter binding or higher affinity. Km does not directly report maximum velocity or cofactor requirements.

Question 24Molecular and Cell Biology

A eukaryotic cell is exposed to a toxin that prevents the ribosome from releasing the completed polypeptide. Which cellular process is most directly disrupted?

  1. Replication of DNA before mitosis
  2. Modification of lipids in the smooth ER
  3. Translation of mRNA into protein Correct answer.
  4. Transcription of DNA into mRNA

Explanation

Ribosomes translate mRNA into polypeptides and must release the finished chain for proper folding and function. Blocking release halts protein production at the translation stage. Transcription, replication, and lipid modification occur through different machinery.

Question 25Molecular and Cell Biology

A cell membrane is experimentally enriched with cholesterol at low temperature. What effect is most expected?

  1. The membrane loses its proteins because cholesterol displaces them
  2. The membrane becomes less fluid at low temperature because cholesterol restricts phospholipid movement Correct answer.
  3. The membrane allows all ions to pass freely through the cholesterol
  4. The membrane becomes more fluid because cholesterol breaks apart the bilayer

Explanation

At low temperatures, phospholipids pack closely and membranes risk becoming too rigid. Cholesterol inserts between tails and prevents tight packing, but in this low-temperature context it still restricts excessive movement, keeping the membrane from becoming overly rigid or crystalline.

Question 26Molecular and Cell Biology

Which molecule stores the genetic instructions that are transcribed into mRNA in a eukaryotic cell?

  1. Cholesterol
  2. Cellulose
  3. ATP
  4. DNA Correct answer.

Explanation

DNA carries the base sequence that is copied into messenger RNA during transcription, and it remains in the nucleus of a eukaryotic cell. ATP is an energy carrier, cholesterol is a membrane lipid, and cellulose is a structural polysaccharide, so none of them stores genetic instructions.

Question 27Molecular and Cell Biology

Which part of a eukaryotic cell contains most of the cell's DNA?

  1. The lysosome
  2. The smooth endoplasmic reticulum
  3. The Golgi apparatus
  4. The nucleus Correct answer.

Explanation

The nucleus is bounded by a double membrane and holds the chromosomes, so it contains most of the cell's DNA. The Golgi apparatus, lysosomes, and smooth endoplasmic reticulum are membrane-bound organelles that modify, digest, or synthesize materials rather than store chromosomes.

Question 28Molecular and Cell Biology

Which structure is the site of protein synthesis in a cell?

  1. The mitochondrion
  2. The ribosome Correct answer.
  3. The central vacuole
  4. The cytoskeleton

Explanation

Ribosomes bind mRNA and use transfer RNA to assemble amino acids into a polypeptide chain. Mitochondria release energy through respiration, the central vacuole stores water and solutes, and the cytoskeleton provides shape and movement, so none of these builds proteins.

Question 29Molecular and Cell Biology

Which statement correctly describes the role of ATP in a cell?

  1. It releases usable energy when its terminal phosphate bond is broken. Correct answer.
  2. It catalyzes the hydrolysis of proteins in the stomach.
  3. It forms the main structural framework of the plasma membrane.
  4. It permanently stores genetic information in its base sequence.

Explanation

ATP releases energy when its terminal phosphate group is removed, forming ADP and inorganic phosphate, and that energy drives many cellular processes. ATP is an energy carrier rather than a genetic store, a membrane scaffold, or a digestive enzyme.

Question 30Molecular and Cell Biology

Which statement describes the function of the cell membrane?

  1. It regulates which substances enter and leave the cell. Correct answer.
  2. It packages proteins into vesicles for export.
  3. It synthesizes ribosomal RNA from a DNA template.
  4. It breaks down fatty acids for energy production.

Explanation

The plasma membrane is selectively permeable, so it controls the movement of ions and molecules into and out of the cell. Ribosomal RNA synthesis occurs in the nucleolus, protein packaging occurs in the Golgi apparatus, and fatty acid breakdown occurs mainly in mitochondria.

Question 31Molecular and Cell Biology

Which property of water allows it to dissolve many ionic and polar substances?

  1. Its molecules are polar and form hydrogen bonds with charged particles. Correct answer.
  2. It forms covalent bonds with dissolved ions.
  3. It has a very low specific heat capacity.
  4. Its molecules are nonpolar and repel ions.

Explanation

Water is a polar molecule with partial charges that attract ions and other polar molecules, surrounding them and keeping them in solution. Water is not nonpolar, it has a high specific heat, and it interacts with dissolved ions through attractions rather than covalent bonds.

Question 32Molecular and Cell Biology

Which of these is a monomer used to build a carbohydrate polymer such as starch?

  1. Nucleotide
  2. Glucose Correct answer.
  3. Amino acid
  4. Fatty acid

Explanation

Starch is a polymer of glucose units joined by glycosidic bonds. Amino acids build proteins, nucleotides build nucleic acids, and fatty acids build lipids, so none of those monomers forms starch.

Question 33Molecular and Cell Biology

Which statement best describes the difference between a prokaryotic cell and a eukaryotic cell?

  1. A prokaryotic cell contains chromosomes, while a eukaryotic cell contains none.
  2. A prokaryotic cell lacks a membrane-bound nucleus, while a eukaryotic cell has one. Correct answer.
  3. A prokaryotic cell has a nucleus, while a eukaryotic cell lacks one.
  4. A prokaryotic cell has mitochondria, while a eukaryotic cell lacks them.

Explanation

Prokaryotic cells keep their DNA in a nucleoid region without a nuclear membrane, while eukaryotic cells enclose their chromosomes in a nucleus. Both cell types can carry chromosomes, and mitochondria are typical of eukaryotic cells rather than prokaryotic ones.

Question 34Molecular and Cell Biology

A student examines a cell and finds that it has a cell wall made of cellulose, chloroplasts, and a large central vacuole. Which type of cell is being observed?

  1. An animal cell
  2. A plant cell Correct answer.
  3. A fungal cell
  4. A bacterial cell

Explanation

Cellulose cell walls, chloroplasts, and a large central vacuole are characteristic features of plant cells. Animal cells lack cell walls and chloroplasts, bacterial cell walls are not made of cellulose, and fungal cell walls are made of chitin.

Question 35Molecular and Cell Biology

A red blood cell is placed in distilled water. Which change is most likely to occur?

  1. The cell loses its membrane and becomes a naked nucleus.
  2. The cell swells and may burst as water enters. Correct answer.
  3. The cell keeps its original volume because water cannot cross the membrane.
  4. The cell shrinks as water leaves.

Explanation

Distilled water is hypotonic relative to the cytoplasm, so water moves inward by osmosis and the cell swells and may lyse. Water crosses the membrane freely, and the cell does not shrink or lose its membrane in a hypotonic solution.

Question 36Molecular and Cell Biology

A protein is moved from the rough endoplasmic reticulum to the Golgi apparatus and then secreted from the cell. Which sequence correctly traces this pathway?

  1. Smooth ER, then mitochondrion, then transport vesicle, then plasma membrane
  2. Golgi apparatus, then rough ER, then lysosome, then plasma membrane
  3. Rough ER, then nucleus, then Golgi apparatus, then plasma membrane
  4. Rough ER, then Golgi apparatus, then transport vesicle, then plasma membrane Correct answer.

Explanation

Secretory proteins are synthesized on the rough ER, modified as they pass through the Golgi apparatus, and then carried in vesicles that fuse with the plasma membrane. The nucleus, lysosomes, and mitochondria are not steps in this export pathway.

Question 37Molecular and Cell Biology

A cell is treated with a chemical that stops the electron transport chain from pumping hydrogen ions. What is the most direct effect on ATP production?

  1. ATP production increases because glycolysis is bypassed.
  2. ATP production by oxidative phosphorylation decreases because the proton gradient is not maintained. Correct answer.
  3. ATP production is unchanged because ATP is made directly from glucose.
  4. ATP production stops completely because substrate-level phosphorylation also requires the proton gradient.

Explanation

Oxidative phosphorylation depends on a proton gradient built by the electron transport chain, so blocking proton pumping reduces ATP synthase activity. Glycolysis and the Krebs cycle still make a small amount of ATP by substrate-level phosphorylation, which does not require the proton gradient.

Question 38Molecular and Cell Biology

In a fermentation experiment, yeast cells are kept in an oxygen-free environment. Which product builds up as pyruvate is converted to regenerate NAD+?

  1. Acetyl-CoA and citrate
  2. Glucose and water
  3. Ethanol and carbon dioxide Correct answer.
  4. Lactic acid and oxygen

Explanation

When oxygen is unavailable, yeast convert pyruvate to ethanol and carbon dioxide, which regenerates NAD+ so glycolysis can continue. Lactic acid fermentation occurs in some animal cells and bacteria, while acetyl-CoA and citrate are intermediates of aerobic respiration.

Question 39Molecular and Cell Biology

A gardener waters a plant with a fertilizer solution that is more concentrated than the fluid inside the root cells. Which outcome is most likely for the roots?

  1. Water moves equally in both directions, so the plant is unaffected.
  2. Water leaves the root cells, causing the plant to wilt. Correct answer.
  3. Water enters the root cells, causing them to burst.
  4. Salts move into the root cells until the concentrations are equal.

Explanation

A hypertonic fertilizer solution has a higher solute concentration than the root cell fluid, so water moves out of the cells by osmosis and the plant wilts. Water does not move equally in both directions when a concentration gradient exists, and the fertilizer salts may not cross the membrane freely.

Question 40Molecular and Cell Biology

A drug prevents the mitotic spindle from attaching to chromosomes. Which outcome is most likely?

  1. Chromosomes fail to separate properly, so daughter cells may receive unequal numbers of chromosomes. Correct answer.
  2. Chromosomes condense into chromatin and the cell reenters interphase normally.
  3. The cell membrane dissolves, preventing cytokinesis.
  4. DNA replication stops before the cell enters mitosis.

Explanation

Spindle fibers attach to chromosomes and pull them apart during anaphase, so blocking attachment causes unequal chromosome distribution. DNA replication occurs before mitosis and is not directly stopped by this drug, and the cell membrane is not dissolved during normal division.

Question 41Molecular and Cell Biology

A mutation changes a single DNA base, and the resulting codon now specifies a different amino acid. Which term best describes this mutation?

  1. Nondisjunction
  2. Frameshift mutation
  3. Silent mutation
  4. Missense mutation Correct answer.

Explanation

A missense mutation changes one base so that a codon codes for a different amino acid. A silent mutation leaves the amino acid unchanged, a frameshift shifts the reading frame, and nondisjunction is a failure of chromosomes to separate during meiosis.

Question 42Molecular and Cell Biology

A cell is placed in a solution in which the solute concentration outside the cell is lower than inside the cell. Which statement describes the net movement of water?

  1. Water moves out of the cell because the outside solution is hypertonic.
  2. Water does not move because the solute cannot cross the membrane.
  3. Water moves out of the cell because the outside solution is isotonic.
  4. Water moves into the cell because the outside solution is hypotonic. Correct answer.

Explanation

A solution with a lower solute concentration than the cell is hypotonic, so water moves inward by osmosis. Water moves out in a hypertonic solution, and in an isotonic solution net movement is roughly balanced.

Question 43Molecular and Cell Biology

An enzyme that normally works in the small intestine at about pH 8 is placed in a strongly acidic buffer. Which change is most likely?

  1. The enzyme's shape changes, reducing its ability to bind the substrate. Correct answer.
  2. The enzyme becomes more efficient because acid always increases enzyme activity.
  3. The enzyme begins to synthesize DNA instead of catalyzing its normal reaction.
  4. The enzyme's activation energy requirement disappears.

Explanation

Enzymes have an optimal pH, and strong acid can alter the shape of the active site so the substrate no longer binds efficiently. Acid does not remove the activation energy requirement, change the enzyme's function into DNA synthesis, or universally increase activity.

Question 44Molecular and Cell Biology

A segment of DNA has the template sequence 3'-TAC GGA-5'. Which mRNA sequence is transcribed from this template?

  1. 5'-UAC GGA-3'
  2. 5'-TAC GGA-3'
  3. 5'-AUG GGU-3'
  4. 5'-AUG CCU-3' Correct answer.

Explanation

RNA polymerase pairs adenine with thymine, uracil with adenine, guanine with cytosine, and cytosine with guanine, so the template TAC GGA yields AUG CCU. The mRNA also uses uracil instead of thymine, which rules out options containing T.

Question 45Molecular and Cell Biology

In photosynthesis, a plant is supplied with water containing labeled oxygen. Where does the labeled oxygen first appear?

  1. In carbon dioxide released by cellular respiration
  2. In ATP produced by the electron transport chain
  3. In oxygen gas released during the light-dependent reactions Correct answer.
  4. In glucose produced by the Calvin cycle

Explanation

Water is split during the light-dependent reactions, and the oxygen atoms from water form O2 gas that is released. The carbon in glucose comes from carbon dioxide, and ATP does not contain the oxygen atoms from water.

Question 46Molecular and Cell Biology

Two enzymes act on the same substrate, but enzyme A has a much lower Michaelis constant (Km) than enzyme B. What does this difference indicate about enzyme A?

  1. It reaches half of its maximum rate at a lower substrate concentration. Correct answer.
  2. It cannot be saturated by any substrate concentration.
  3. It has a higher maximum reaction rate than enzyme B.
  4. It requires a higher substrate concentration to reach half of its maximum rate.

Explanation

Km is the substrate concentration at which an enzyme reaches half of its maximum rate, so a lower Km means half-maximal rate is reached at a lower substrate concentration, indicating higher affinity. Km does not directly report the maximum rate, which is given by Vmax.

Question 47Molecular and Cell Biology

A scientist finds an organic molecule that contains carbon, hydrogen, oxygen, nitrogen, and sulfur, and that is built from amino acid subunits joined by peptide bonds. Which class of biological macromolecule is this?

  1. Nucleic acid
  2. Lipid
  3. Carbohydrate
  4. Protein Correct answer.

Explanation

Proteins are polymers of amino acids linked by peptide bonds, and sulfur appears in the side chains of certain amino acids such as cysteine and methionine. The combination of nitrogen and sulfur together with amino acid subunits identifies a protein.

Question 48Molecular and Cell Biology

Which statement correctly distinguishes a dehydration synthesis reaction from a hydrolysis reaction?

  1. Dehydration synthesis splits a polymer by adding water, while hydrolysis joins monomers by removing a water molecule.
  2. Dehydration synthesis joins monomers without producing water, while hydrolysis joins monomers and releases water.
  3. Dehydration synthesis and hydrolysis both break polymers apart, but only hydrolysis requires an enzyme.
  4. Dehydration synthesis joins monomers by removing a water molecule, while hydrolysis splits a polymer by adding water. Correct answer.

Explanation

In dehydration synthesis, a hydroxyl group from one monomer and a hydrogen from another are removed as water, forming a covalent bond that links the monomers. Hydrolysis reverses this process, using a water molecule to break the bond between subunits.

Question 49Molecular and Cell Biology

A student uses a microscope and sees a cell with a rigid outer layer, circular DNA not enclosed by a membrane, and ribosomes but no mitochondria. Which conclusion is best supported?

  1. The cell is an animal cell, because it lacks mitochondria.
  2. The cell is a virus, because it lacks membrane-bound organelles.
  3. The cell is a prokaryote, such as a bacterium. Correct answer.
  4. The cell is a plant cell, because it has a rigid outer layer.

Explanation

Circular DNA that is not enclosed in a nucleus and the absence of membrane-bound organelles are defining features of prokaryotic cells. The rigid outer layer is a cell wall, which bacteria can also possess.

Question 50Molecular and Cell Biology

Which part of a eukaryotic cell controls which substances enter and leave the cell and separates the cytoplasm from the external environment?

  1. The nucleus
  2. The Golgi apparatus
  3. The ribosome
  4. The plasma membrane Correct answer.

Explanation

The plasma membrane is a phospholipid bilayer with embedded proteins that forms the outer boundary of the cell and regulates the movement of materials across it. Its selective permeability allows the cell to maintain a distinct internal environment.